Transmission of optical data at high bit rates, e.g. 40 Gb/s, in general benefits from the use of narrow pulse width optical data. Narrower temporal pulse width means a broader frequency spectrum for each data pulse, which limits the number of channels that can be used in a WDM scheme. For example, a Return to Zero (RZ) pulse at 10 Gb/s will have a spectral width of about 40 Ghz whilst an RZ pulse at 40 Gb/s will have a spectral width of about 160 GHz.
WDM schemes increase fibre capacity by transmitting multiple channels, each at different wavelengths, over a single fibre. However, optically amplified systems have a limited useable bandwidth. Using a higher bit rate increases the bandwidth of each channel and therefore reduces the number of channels which can be used. The result is that a bit rate of 40 Gb/s offers no advantage over a lower bit rate of say 10 Gb/s because the maximum amount of information that can be transmitted in a given time over an optical fibre link at each rate is approximately the same.
There are a number of schemes which have been developed to improve spectral efficiency at high bit rates, such as using vestigial sideband (VSB) filtering and polarisation division multiplexing, all involving further processing of the optical data signals. The aim of the present invention is to provide a data format, and a transmitter and method for producing the same, which provides improved spectral efficiency over traditional data formats.